Axial fan assemblies are known in the art and are essentially a propeller-type impeller rotatably supported within an annular housing. The diameter of the annular housing is sized to provide a small tip clearance from the end of the propeller-type impeller and provides an air passageway from an inlet end to an outlet end of the housing. Rotation of the impeller causes air to enter the inlet end, pass along the air passageway and across the impeller, and exit the outlet end of the housing.
Axial fan assemblies are employed for a variety purposes and in various environments. One particular application of axial fan assemblies is for providing air flow in air-cooled heat exchanger systems (e.g. fin-fan coolers and steam condensers). For example, air-cooled steam condensers are used in electric power generation plants. In these plants axial fan assemblies are employed to pass ambient air across heat exchangers to condense turbine exhaust steam exiting the final stages of expansion processes in the power plant into liquid water to be reused in the plant.
U.S. Pat. Nos. 8,302,670 and 8,776,545 and US Patent Application Publication No. 2005/0006050, which all are incorporated herein by reference in their entirety for all purposes, describe uses of axial fan assemblies disposed in arrays for heat exchange with steam exiting power generation plants (e.g. air-cooled condensing systems, or ACCs). Typically, at electric power generation plants that employ air-cooled condensers, a plurality of axial fan assemblies are configured in an array located outside of the plant. Turbine exhaust steam from the plant is piped to heat exchangers positioned adjacent to and/or above the outlet ends of housings of the assemblies in the array. The array of axial fans and associated heat exchangers are supported by a support structure so that the axes of the assemblies are vertical with the inlets pointed toward and positioned between about 20 and 150 feet from the ground. Ambient outside air is caused to flow in a vertical direction across the heat exchanger(s) by each axial fan assembly disposed in the array, thereby cooling the steam in the heat exchanger and condensing it into liquid water, which is piped back to the plant for reuse in the electric power generation process.
Problems with axial fan assemblies are well known in the art. For example, when axial fan assemblies are employed in air-cooled condensing systems at electric power generation plants, the cooling load is supplied by ambient air from the outside environment. Thus, the axial fan assemblies performance, and the plant's performance is largely dependent upon environmental conditions, such as ambient temperature, outside of the plant. Additionally, due to the size of the propeller-type impellers of these assemblies (e.g. 5′-20′, or more, per radial blade) and the RPMs (e.g. from about 10-200 RPMs, for example about 100 RPM) required to produce satisfactory air flow across the heat exchangers, if a blade breaks during operation, it can become a missile with a potential projectile path of any of the 360°s in its path of rotation. If a blade breaks it could be thrown into and damage an adjacent axial fan assembly in the array or be thrown elsewhere and cause damage to person or other property. Furthermore, the vibration caused by the unbalanced forces in the fan with the broken blade often destroys the remaining blades in the particular assembly. Damage to one or more axial fan assemblies requires shut down of the respective assembly, reduction in electric power output, and potentially shut down of the entire electric power generation plant. The present invention provides solutions to these and other problems.